From U.S. Pat. No. 5,353,585, a method has become known in which atomized water is introduced into the induced air flow before the compressor of a gas turbine group. Due to evaporation cooling, the density of the induced air is increased and, with it, the mass flow of the working fluid and the maximum attainable power of the gas turbine group.
In U.S. Pat. No. 6,216,443, injecting water is proposed in such a way that liquid droplets enter the compressor. The droplets evaporate within the compressor, during the compression process, and this leads to intensive internal cooling of the compressor. In addition to the effect of increasing the mass flow, the evaporation cooling before the compressor also decreases the power taken by the compressor and the compressor exit temperature is reduced so that it is possible to increase the amount of fuel burnt in a subsequent combustion chamber. Overall, this results in an increase in the attainable maximum power of the gas turbine group, which maximum power is, of course, mainly limited by the mass flow of the working fluid and the permissible turbine inlet temperature.
From U.S. Pat. No. 6,012,279, furthermore, it is known to inject liquid between two partial compressors. This likewise effects a cooling of the working fluid, on the one hand between the compression steps in the first and second partial compressors and, on the other hand, also during the second compression step.
In the specialist world, the fluid injection in a compressor is currently referred by different concepts, such as “high fogging”, “overfogging”, “wet compression” and “overspray cooling”. It should be mentioned that the positive effect has, in any case, been known per se for a long time, as is indicated for example by FR 1,563,749. The evaporation of a liquid in the working fluid during the compression offers a quite good approximation to an isothermal compression for comparatively small requirements in terms of apparatus.
In order to cool the working fluid, it is also known to use heat exchangers as intercoolers or, indeed, to use refrigeration machines for cooling the induced air.
Intercoolers in compressors are frequently operated at full cooling power over the whole of the operating range. Modern gas turbine groups operate far from the per se most favorable design point over wide load ranges.
U.S. Pat. No. 6,216,443 proposes that the liquid injection should only be put into operation when the gas turbine group, without water injection, is already being operated at full load and when the required power is still higher than the useful power which is actually delivered. This means that the gas turbine group itself is being operated with the maximum inlet volume flow of the compressor and the turbine entry temperature is at the maximum permissible value. The cooling of the working fluid, by the injection of liquid into the compressor, is used for a further increase in power. In the method proposed there, the necessary mass flow of the injection water is initially determined and injected as a function of the additional useful power which has to be provided. The gas turbine group therefore deviates from the full-load point and the fuel supply is successively increased until the gas turbine group is again operated at the full-load condition, which means with maximum permissible turbine inlet temperature and, if appropriate, completely opened inlet guide vane row.
In practice, however, it is found that there can be a duration of some 0.5 to 2 seconds before the water droplets have passed from the injection location to the compressor inlet. Likewise, the transit times of the components in the regulation and control mechanisms frequently used in gas turbine groups delay the detection of the temperature changes effected by the water injection and also, therefore, their effect on the fuel supply. In consequence, it is found that the water injection only becomes effective with a time lag. Similarly, it is impossible to realize an arbitrarily rapid switching on of the injected water quantity and an arbitrarily rapid increase of it. In addition, it is found that the dead periods and delay times, as described above, of a control line reduce the stability of a control circuit in such a way that the possible reaction speed is greatly reduced. Summarizing, therefore, the use of measures for cooling the working fluid during the compression and, in particular, the injection of water before the compressor, cannot be used to an unlimited extent for rapid increases in power.